Since a lithium ion battery can achieve high energy density, it attracts attention as a power source for cellular phones and notebook personal computers, a large-scale power source for electric power storage, and a power source for motor vehicles.
Although a lithium ion battery can achieve high energy density, higher safety is required when it is increased in size. For example, a very high safety is required for the large-scale power source for electric power storage and the power source for motor vehicles, and although measures such as a structural design of a cell, a package, and the like, a protective circuit, an electrode material, an additive having an overcharge-preventing function, and the strengthening of a shutdown function of a separator have been taken as safety measures, thus giving sufficient consideration to safety and ensuring safety, one of the means for further increasing safety is the flame retardation of an electrolyte.
A lithium ion battery uses an aprotic solvent such as cyclic carbonate and linear carbonate as an electrolyte solvent, and these carbonates are characterized in that they have a high dielectric constant and high ionic conductivity of lithium ions, but have a low flash point and are flammable. Generally, cyclic carbonates are characterized in that they have a high dielectric constant and high viscosity, while linear carbonates are characterized in that they have a low dielectric constant and low viscosity. Therefore, these solvents are mixed for use in a lithium ion battery in accordance with the applications thereof.
On the other hand, a research of using as an electrolyte solvent an ionic liquid which assumes a liquid state at a certain temperature has been made. Since an ionic liquid is characterized in that it has very low inflammability because it does not have volatility and has high decomposition temperature, a research using an ionic liquid as an electrolyte of a lithium ion battery has been actively done.
In Patent Literature 1, an ionic liquid containing a 1-methyl-3-ethylimidazolium cation is used as an electrolyte solvent, and since this electrolyte solvent does not volatilize even under a high temperature environment of 120° C., it shows good characteristics. However, the ionic liquid containing this cation has low reduction stability and undergoes reductive decomposition at a potential of 1 V or less to Li/Li+. Therefore, there has been a problem that the cycle characteristics of a battery are significantly reduced when a negative electrode is activated at 1 V or less to Li/Li+. Therefore, it is necessary to use a negative electrode active material in which the action potential of the negative electrode is 1 V or more to Li/Li+, and since the battery voltage is reduced in this case as compared with the case where a carbon negative electrode is used, high energy density is not obtained.
Patent Literature 2 describes that an ionic liquid having improved reduction stability which comprises at least one cation selected from the group consisting of N-methyl-N-ethyl pyrrolidinium, N-methyl-N-propyl pyrrolidinium, N-methyl-N-ethyl pyrrolidinium, and N-methyl-N-propyl piperidinium has excellent reduction stability even when the action potential of Li metal, Sn, or the like is 1 V or less to Li/Li+, and that the characteristics of a battery in which Li metal is used as the negative electrode and LiCoO2, is used as the positive electrode include high energy density and excellent storage characteristics and flame retardancy.
Patent Literature 3 discloses a 4 V class lithium secondary battery using an ionic liquid comprising a bis(fluorosulfonyl)imide anion, wherein a negative electrode active material in which insertion and elimination of Li are possible at a potential close to the oxidation-reduction potential of Li metal, for example, graphite, tin oxide, or a Si-based material such as SiO2 is used.
Further, Non-Patent Literature 1 describes that an ionic liquid comprising a bis(fluorosulfonyl)imide anion allows insertion and elimination of Li ions on graphite.
However, Patent Literature 3 and Non-Patent Literature 1 only describe that charge and discharge is possible when graphite is used, but there has been a problem that sufficient capacity, rate characteristics, and cycle characteristics are not obtained because an ionic liquid with high viscosity has low impregnation into graphite.
On the other hand, although a carbon material is generally used as a negative electrode material of a lithium ion battery, it is known that a carbonate such as propylene carbonate in an electrolyte solvent undergoes reductive decomposition at about 1 V to Li/Li+ on the surface of graphite having high crystallinity to increase irreversible capacity to reduce charge/discharge efficiency and cycle characteristics. It is known that, on the surface of carbon having a very high degree of graphitization, a cyclic carbonate such as PC (propylene carbonate) is easily decomposed to cause reduction in cycle characteristics.
There has been a problem that an ionic liquid with high viscosity generally has a low impregnation into a porous material such as an electrode and a separator. In order to improve the impregnation into a porous material of an ionic liquid, a technique of mixing a carbonate to reduce the viscosity has been studied, for example, in Patent Literatures 4 and 5. Patent Literature 4 describes that a cyclic carbonate and/or a linear carbonate is mixed in an amount of 0.1 to 30% by volume, and Patent Literature 5 describes that a cyclic carbonate and/or a linear carbonate is mixed in an amount of 50% by volume or more. It is shown that mixing a cyclic carbonate and/or a linear carbonate having a lower viscosity than an ionic liquid reduces the viscosity of an electrolyte solvent, improves the impregnation into a porous material such as an electrode and a separator, and improves energy density. However, a cyclic carbonate has low reduction stability and is particularly apt to undergo reductive decomposition on the surface of graphite. Therefore, there has been a problem that the carbonate undergoes reductive decomposition on the surface of graphite while repeating the cycle, and characteristics such as cycle characteristics and storage characteristics are significantly reduced. Further, there has been a problem that even when an ionic liquid having low reduction stability is used, the ionic liquid undergoes reductive decomposition while repeating the cycle to significantly reduce battery characteristics.
A technique is known in which there is used, as an additive, a substance which undergoes reductive decomposition at a potential higher than a carbonate used as an electrolyte solvent to produce a protective film having high lithium ion permeability, SEI (Solid Electrolyte Interface). It is known that the control of the protective film is indispensable to achieve high performance of a negative electrode because the protective film has large influence on charge/discharge efficiency, cycle characteristics, and safety, and with respect to a carbon material and an oxide material, reduction in the irreversible capacity thereof is required.
Then, it is shown that irreversible capacity is reduced and capacity, cycle characteristics, and the like can be improved, while holding the flame retardancy of the electrolyte, by further incorporating an additive for forming a protective film on the surface of graphite. The following is open to the public as a technique using graphite. It is shown that the above improvement can be made by incorporating a cyclic ester having a π-bond such as vinylene carbonate, in Patent Literature 6; by incorporating a cyclic organic compound having an S═O bond such as 1,3-propane sultone, in Patent Literature 7; by incorporating a cyclic carbonate having a C═C unsaturated bond such as vinylethylene carbonate, in Patent Literature 8; and by incorporating a cyclic organic compound having an S═O bond such as 1,3-propane sultone and/or a cyclic carbonate having a π-bond such as vinylene carbonate, in Patent Literature 9.
However, since graphite has very high activity to decompose an electrolyte, it is necessary to add a large amount of protective film-forming substance as described in Patent Literatures 6 to 9, in order to form a protective film for obtaining good characteristics over a long period of time. There has been a problem that, when a large amount of additives is used, battery characteristics are reduced and charge/discharge efficiency is reduced due to the increase in resistance or the increase in irreversible capacity. Further, in Patent Literature 10, there is disclosed a technique of a negative electrode active material comprising a carbon material (hardly graphitizable carbon) in which the spacing of the (002) plane is 0.34 nm or more.